The present invention relates to a hydrophilic coating composition for aluminum members such as heat exchanger fins, etc., and a bath containing such a hydrophilic coating composition. More particularly, it relates to a hydrophilic coating composition for imparting a good corrosion resistance and a high hydrophilic nature to the aluminum members such as fins of heat exchangers while reducing unpleasant odor which are generated from the heat exchangers, and a bath containing such a hydrophilic coating composition.
Since aluminum and its alloys are light and have good workability and heat conductance, they are widely used for fins of heat exchangers. Recently more and more air conditioners have been used not only for cooling but also for warming and dehumidification. In heat exchanger parts of these air conditioners, aluminum alloy fins are generally used.
However, it is observed that moisture tends to be condensed and deposited as water droplets on the fin surfaces of air conditioners during cooling operations. If the fin surface is water-repellent, this condensed water tends to be deposited in a hemispherical form on the fin surface or forms bridges between the fins, preventing smooth air flow, which in turn increases resistance of air flow, thereby decreasing heat exchange efficiency.
In addition, although aluminum and its alloys are essentially excellent in corrosion resistance, it is likely that the condensed water remaining on the aluminum fin surfaces for a long period of time functions like an oxygen concentration cell, and that contaminants in the air are absorbed and concentrated in the condensed water. As a result, a hydration reaction and a corrosion reaction are accelerated. Those products produced by the corrosion are accumulated on the aluminum fin surfaces, which not only deteriorate heat exchange performance, but also are blown out of the air conditioners as white fine powders together with a hot air during the warming operation in the winter season. This also causes an unpleasant odor.
To solve these problems, various attempts have been made to form coatings for aluminum fin surfaces to improve their corrosion resistance and hydrophilic nature.
For instance, Japanese Patent Laid-Open No. 1-299877 discloses a surface treatment agent comprising fully-saponified polyvinyl alcohol, a water-soluble polyamide resin and a urea resin at a weight ratio of 1:0.5-2 0.01-0.5, and water. A bath containing 2-10 weight %, on a solid basis, of the surface treatment agent is used for surface treatment of the aluminum fins of heat exchangers.
Japanese Patent Laid-Open No. 1-240688 discloses an aluminum fin having a corrosion-resistant chemical primer coating and a hydrophilic coating comprising a benzimidazole compound.
Further, Japanese Patent Laid-Open No. 3-49944 discloses a heat exchanger made of aluminum and comprising (a) tubes made of aluminum through which a fluid flows, (b) fins made of aluminum and arranged between the tubes for accelerating heat exchange between the fluid and air, (c) a first protective chemical coating layer formed on the tubes, (d) a second protective layer formed on the first protective layer containing an antimicrobial agent such as 2,2-dithiobis(pyridine-1-oxide).
However, any one of the coatings disclosed in the above references fails to show a good hydrophilic nature and a sufficient effect in preventing unpleasant odor. The conventional surface treatment technologies for heat exchangers, etc. fail to provide coatings which have not only sufficient hydrophilic nature, corrosion resistance and strength but also excellent odor-preventing effect.